One of the problems associated with threaded fasteners is the accidental disassembly when pre-load is lost. When pre-load is lost, a standard fastener quickly vibrates out, causing the assembly to loosen. ©1993-2006 Long-Lok Fasteners Corporation.
A threaded fastener of the prevailing torque type is frictionally resistant to rotation due to a built-in wedge. Such fastener retains its locking ability independent of axial tension or pre-load. Self-locking fasteners of this type were developed to retain the advantage of reusability while preventing the problems of accidental disassembly when pre-load is lost. ©1993-2006 Long-Lok Fasteners Corporation.
Jam nuts, cotter pins, lock nuts, lock washers and similar devices also prevent the loss of the bolt or nut by back off but they result in added weight, inconvenience and cost. They do not reduce the tendency to fatigue when loose. Further, the insecurity of conventional mechanical locks is reason enough for most designers to reject them. Such insecurity arises from the frequency of split-type washers breaking, damage to surface areas caused by external locking devices, and the ineffectiveness of such devices when adjustments are needed. ©1993-2006 Long-Lok Fasteners Corporation.
A prevailing torque type fastener or self-locking fastener was developed to retain the advantage of reusability while preventing the problems of accidental disassembly when pre-load is lost. Self-locking fasteners virtually eliminate the possibility of a bolted assembly coming apart during operation. To achieve this benefit, the self-locking fastener has to be properly designed, engineered and installed. Self-locking helps maintain a tight joint and also helps prevent fatigue failure in the joint. Self-locking fasteners resist rotation on the first installation and on subsequent installations and removals. In addition, self-locking fasteners also decrease the tendency of the fastener to fatigue by reducing the vibration transferred to the fastener. ©1993-2006 Long-Lok Fasteners Corporation.
One of the earliest methods of producing prevailing torque type locking features (also referred to as “patches” in the art) on externally threaded fasteners was the “spray-on” nylon patch. In this type of patch, nylon resin is deposited on the threads of screws, which had been pre-heated to a temperature range slightly above the melting point of the plastic powder, thus allowing the plastic powder to melt rapidly upon contact with the hot threads. This mass of molten plastic was then “quenched” in a water/oil medium to cool the screws, leaving a deposit of plastic firmly attached within a predetermined location of the screw threads. This predetermined deposit of plastic is referred to as a “patch.”
To produce an effective patch the following principles must be considered: 1) transferring the plastic resin to the hot screws, 2) controlling the amount of powder to be deposited (patch geometry), and 3) controlling the patch location on the screw.
Many techniques and methods have been employed over the years to apply plastic nylon resin to the hot screws in order to produce “locking” features or “patches”. Some obvious strategies included incorporating “gravity” and letting powder free-fall over the hot screw threads. Others have utilized shaking bars, spray bars, and spray nozzles powered by compressed air. Due to the uncontrollable nature of the propelled powder, devices, such as barriers, templates, and spray tips, have been utilized. One method uses nozzles made from standard elongated copper tubing in an attempt to force the powder spray pattern to conform to the screw thread profile.
In order to provide some consistency in the prevailing torque characteristics, control of the deposited plastic resin on the screw must be achieved. Many techniques have been employed to control the location and geometry of the patches. The technique typically involves strategically located “jets” of air to prevent powder from contacting the hot screw heads in areas where powder is unwanted (e.g. lead-in). The amount of patch material to be deposited is determined by the volume of the powder contacting the threads and melting in place. The circumferential coverage is not typically considered a critical characteristic of the patch, so the patch may typically range from 60° to a full 360° circumferential coverage. The typical specification limit on the height of the patch over the major diameter is 0.003 inches.
Rotation of the screws as they pass-by the point of powder application and various air-jets, are typically utilized to control the patch geometry. The results are patches, which will yield prevailing torque within a pre-specified range.
Economics dictates what method is acceptable, since nylon resin powder is expensive and rework is costly. Another interesting dilemma is encountered when attempting to re-use powder that did adhere during initial application. Particles of the powder may have actually melted, but not adhered to the screw. When re-used, multiple melting of the powder has been show to cause degradation of the resin properties. These properties can affect prevailing torque characteristics.